This invention relates to improvements in furnace heating systems employing burner assemblies of the type shown in U.S. Pat. Nos. 3,055,652; 3,172,647; 3,174,735; 3,247,884; 3,280,769; 3,464,682; 3,744,965 and 3,843,317. The use of diffusion air in burner assemblies of the indicated type was developed by Bickley Furnaces Inc. in 1960 (see U.S. Pat. No. 3,055,652). By this arrangement, known as jet firing, the burner assembly can provide controlled jet velocities exceeding five hundred miles per hour. This high jet velocity plus close control of jet temperature assures fast, safe, uniform heating in the furnace. The high jet velocity and close temperature control are obtained by injecting secondary diffusion air into the fully burned combustion mixture in a combustion and mixing chamber, the result of which is a high velocity jet of super-heated gases. All combustion takes place in the combustion and mixing chamber and no flame enters the furnace. Furnaces equipped with this type of burner assembly can be automatically controlled at any temperature between 160.degree. F. and 3272.degree. F. (71.degree. C. to 1800.degree. C.).
The above-described burner arrangement has many resultant advantages in industry wherein the firing process is enhanced by shorter firing times, significantly improved temperature uniformity, and reduced fuel consumption.
With the above-described systems, cold (room temperature) diffusion air is introduced into the burner assembly resulting in a dropping of the temperature of the gases entering the furnace chamber because the cold air is mixed with the hot burned gases from the burners. A good deal of heat is consumed by heating up the cold air to whatever temperature is delivered to the furnace heating chamber. Even though there is an inherent fuel usage with cold diffusion air, in the above-described systems, there has been produced a reduction in the fuel usage per pound of work heated as compared with prior systems because of the improvements in the heat transfer mechanism in the furnace.
In accordance with the heating system of the present invention, it is possible to obtain all the advantages of improved heat transfer in the furnace while, at the same time, reducing the amount of heat consumed by the heating system. In accordance with the furnace heating system of the invention, the diffusion air for the burner assembly of the indicated type is taken from the flue of the furnace and is, therefore, at an elevated temperature so that the amount of heat that goes into raising the diffusion air to the desired heating gas temperature is reduced. In other words, the heating system of the invention involves taking spent gases in the furnace (which are taken from the furnace flue) through a blower and delivering such gases back into the burner tunnel. Thus, diffusion gases at an elevated temperature are being mixed with the products of combustion from the burner. When hot gases are delivered to the burner assembly, in order to get the same heating effect in the furnace as with a cold diffusion air system, it is necessary to reduce the firing rate of the burners. Thus, by using hot diffusion gases, a substantial amount of heat is taken off the combustion system to thereby produce considerable fuel savings.
Briefly stated, the above-described general object of the invention is achieved by the provision of a system for heating a furnace having a heating chamber defined by furnace walls is provided with a plurality of individual burner assemblies mounted on a furnace wall to direct heating gas streams into the heating chamber, each of the burner assemblies including a diffusion chamber, a burner arranged to direct products of combustion through the diffusion chamber into a furnace heating chamber and means for directing the flow of diffusion gases into the diffusion chamber to admix with the burner products of combustion so that the gas stream entering the furnace heating chamber has a greater mass velocity than the burner products of combustion, there being provided a diffusion gas supply means connected to the heating chamber of the furnace for delivering hot gases from the furnace heating chamber to the diffusion chambers of each of the burner assemblies.